VIM : What is the difference between let g: , let b: , etc

I often see in vim plugin something like these :

let g:variable
let b:variable
let l:variable

I made a long research on the vim documentation and on the Internet about these letters 'g', 'b', 'l', but I found noting.

So what is these letters corresponding to ? And what is the complete list of letters ?

Solution 1:

See :help internal-variables

It lists the following types:

                (nothing) In a function: local to a function; otherwise: global 
buffer-variable    b:     Local to the current buffer.                          
window-variable    w:     Local to the current window.                          
tabpage-variable   t:     Local to the current tab page.                        
global-variable    g:     Global.                                               
local-variable     l:     Local to a function.                                  
script-variable    s:     Local to a :source'ed Vim script.                     
function-argument  a:     Function argument (only inside a function).           
vim-variable       v:     Global, predefined by Vim.

Solution 2:

b: local to the current buffer

l: local to a function

g: global

:help internal-variables